Handle segfaults in clang-format vim-integration.

A segfault used to clear the entire buffer. With this change, a message
to report a bug is displayed instead.

This should solve most cases similar to llvm.org/PR14774.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@171480 91177308-0d34-0410-b5e6-96231b3b80d8
Port loop-convert into cpp11-migrate
    
Took existing code from loop-convert tool and made it into a cpp11-migrate
transform. Pattern now set for having transform code in subdirectories. Related
changes:
- Makefile and CMakeLists.txt updated to support source files in
  subdirectories.
- At least one transform must be specified. syntax-only tests removed to
  reflect this.
- TODO: port over loop-convert tests.

Reviewers: klimek, silvas


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@171481 91177308-0d34-0410-b5e6-96231b3b80d8